LIDAR system with a multi-mode waveguide photodetector

ABSTRACT

A light detection and ranging (LIDAR) apparatus is provided that includes an optical source configured to emit an optical beam. The LIDAR apparatus further includes free space optics configured to receive a first portion of the optical beam as a target signal and a second portion of the optical beam as a local oscillator signal, and combine the target signal and the local oscillator signal. The LIDAR apparatus includes a multi-mode (MM) waveguide configured to receive the combined signal.

TECHNICAL FIELD

The present disclosure relates generally to light detection and ranging(LIDAR) that provides simultaneous measurement of range and velocityacross two dimensions.

BACKGROUND

Fast-scanning mirrors are the primary components used to illuminate ascene in most conventional LIDAR systems. One mirror typically scansquickly along the X direction (azimuth), while another mirror scansslowly along the Y direction (elevation). Light emission and detectionfrom target reflections are done coaxially, typically via a single modefiber. The collected light has a measured delay or an altered frequencysignature that is used to extract range, and potentially velocity,information. A 3D point cloud can be established when the point-wisedetected range information is combined with angular position feedbackfrom the scanning mirrors.

To achieve higher frame rates, the mirror's angular velocity isincreased, especially that of the scanner in faster scan direction (Xscanner in our case). When using the mirrors with a high angularvelocity and single-mode fiber-based detection, the target signal fromdistant objects is severely degraded. Signal degradation is mainly dueto the difference in angular position of the scanner mirror from thelaunch time of the optical signal (pulsed or frequency swept) to thecollection time of the same signal from a distant scattering target.This slight angular change causes a walk-off of the target signal at thefiber tip decreasing the coupling efficiency, which manifests itself asa weaker signal detection. Such degradation becomes more severe as thefiber diameter decreases, e.g. a single mode fiber with ˜10 μm diameter,or as the mirror's angular velocity increases.

SUMMARY

The present disclosure includes, without limitation, the followingexample implementations.

Some example implementations provide a light detection and ranging(LIDAR) apparatus including an optical source configured to emit anoptical beam. The LIDAR apparatus includes free space optics configuredto receive a first portion of the optical beam as a target signal and asecond portion of the optical beam as a local oscillator signal andcombine the target signal and the local oscillator signal. The LIDARapparatus may further include a multi-mode (MM) waveguide configured toreceive the combined signal.

Some example implementations provide a method including generating, byan optical source of a light detection and ranging (LIDAR) system, anoptical beam towards a target. The method includes receiving, by theLIDAR system, a target signal associated with a reflection of theoptical beam by the target and a local oscillator signal associated areflection of the optical beam by free space optics. The method furtherincludes combining the target signal and the local oscillator signalinto a multi-mode (MM) waveguide.

These and other features, aspects, and advantages of the presentdisclosure will be apparent from a reading of the following detaileddescription together with the accompanying figures, which are brieflydescribed below. The present disclosure includes any combination of two,three, four or more features or elements set forth in this disclosure,regardless of whether such features or elements are expressly combinedor otherwise recited in a specific example implementation describedherein. This disclosure is intended to be read holistically such thatany separable features or elements of the disclosure, in any of itsaspects and example implementations, should be viewed as combinableunless the context of the disclosure clearly dictates otherwise.

It will therefore be appreciated that this Brief Summary is providedmerely for purposes of summarizing some example implementations so as toprovide a basic understanding of some aspects of the disclosure.Accordingly, it will be appreciated that the above described exampleimplementations are merely examples and should not be construed tonarrow the scope or spirit of the disclosure in any way. Other exampleimplementations, aspects, and advantages will become apparent from thefollowing detailed description taken in conjunction with theaccompanying figures which illustrate, by way of example, the principlesof some described example implementations.

BRIEF DESCRIPTION OF THE FIGURE(S)

Embodiments and implementations of the present disclosure will beunderstood more fully from the detailed description given below and fromthe accompanying drawings of various aspects and implementations of thedisclosure, which, however, should not be taken to limit the disclosureto the specific embodiments or implementations, but are for explanationand understanding only.

FIG. 1 illustrates a LIDAR system according to example implementationsof the present disclosure.

FIG. 2 illustrates aspects of the optical circuits of a scanning systemin accordance with embodiments of the present disclosure.

FIG. 3 illustrates aspects of a LIDAR system in accordance withembodiments of the present disclosure.

FIG. 4 illustrates aspects of a LIDAR system having multiple opticalsources in accordance with embodiments of the present disclosure.

FIG. 5A is an illustration of an example demultiplexer in accordancewith embodiments of the present disclosure.

FIG. 5B is an illustration of an example demultiplexer in accordancewith some embodiments of the present disclosure.

FIG. 6 illustrates aspects of the optical circuits of a scanning system.

FIG. 7A illustrates aspects of a LIDAR system having multiple opticalsources generating multiple optical beams in accordance with embodimentsof the present disclosure.

FIG. 7B illustrates aspects of a LIDAR system having multiple opticalsources generating multiple optical beams in accordance with otherembodiments of the present disclosure.

FIG. 8 depicts a flow diagram of a method for combining a target signaland a local oscillator signal into a multi-mode waveguide in accordancewith implementations of the present disclosure.

DETAILED DESCRIPTION

Example implementations of the present disclosure are directed to animproved scanning LIDAR system. Example implementations of the presentdisclosure are based on a type of LIDAR that uses frequency modulation(FM) and coherent detection to overcome the shortcomings of traditionalLIDAR systems and the limitations of prior FM LIDAR systems.Historically, FM LIDAR systems suffer from significant losses in thebeam's return path; thus, such systems, which are often quite bulky,require a higher average beam output power to measure distancescomparable to time-of-flight (TOF) LIDAR systems. However, the range islimited by the operating distance for eye-safe output powers.

Example implementations of the present disclosure are configured tosimultaneously measure the range and velocity, using coherent detectionand having the added benefit of immunity to crosstalk from other LIDARsystems. Other implementations may be used with incoherent systems toimprove, range, framerate, or detection. Example implementationsminimize optical losses in the beam's return path, thereby increasingthe system's measurement range. Additionally, by using nondegenerateoptical sources, example implementations can leverage mature wavelengthdivision multiplexing (WDM) techniques often used in integrated siliconphotonics, a desired platform due to its compactness and relativestability in varying environmental conditions.

As described above, the decenter at the fiber tip upon return of thetarget signal is the primary source of degradation in couplingefficiency. To mitigate the hampering effect of the decentered returnlight at the fiber tip, a conventional FM LIDAR system may combine alocal oscillator (LO) signal with a target signal into a single-mode(SM) waveguide. The combined signal may then be provided to an opticalphotodetector. In general, the efficiency of combining the target signaland the LO signal is based on the spatial overlap between the LO signaland the target signal on the photodetector. Since the mode fielddiameters of SM waveguides are relatively small, coupling the targetsignal with the SM waveguide is extremely challenging and difficult tomanufacture. Furthermore, the SM waveguide does not compensate fortime-dependent deleterious effects of coaxial beam scanning, such asinsufficient de-scan or signal aberrations.

Example implementations of the present disclosure address the above andother deficiencies by an FM LIDAR system combining the LO signal andtarget signal and providing the combined signal to a multi-mode (MM)waveguide. The polarization state of light leaving the system can betransformed with a polarization wave plate of free space optics. Afterthe polarization wave plate, a portion of the light can be reflectedback toward the system as a LO signal, while the remaining light travelsto the environment and may be reflected back by one or more objectswithin system's field-of-view (FOV) as a target signal. The free spaceoptics may be configured to combine the LO signal and the receivedtarget signal to generate a combined signal. In the free space optics,the target signal interferes with the LO signal to form the combinedsignal. Since the polarization state of the combined signal istransformed, the combined signal can be reflected by a polarization beamsplitter to one or more MM waveguides having a larger mode area relativeto a SM waveguide. The combined signal can then be provided to one ormore waveguide photodetectors (WGPDs).

Accordingly, by providing the combined signal to a MM waveguide, theperformance of the FM LIDAR system is improved. Since the MM waveguideshave a larger mode area when compared to SM waveguides, coupling thecombined signal with the MM waveguide is more efficient when compared toconventional FM LIDAR systems utilizing an SM waveguide, improving theperformance and manufacturing capabilities of the FM LIDAR system.Furthermore, using an MM waveguide to receive the combined signal canhelp compensate for insufficient de-scan effects and aberrations thatmay be inherent in a coaxial LIDAR system, further improving theperformance of the FM LIDAR system.

FIG. 1 illustrates a LIDAR system 100 according to exampleimplementations of the present disclosure. The LIDAR system 100 includesone or more of each of a number of components, but may include fewer oradditional components than shown in FIG. 1 . The LIDAR system 100 may beimplemented in any sensing market, such as, but not limited to,transportation, manufacturing, metrology, medical, and security systems.For example, in the automotive industry, the described beam deliverysystem becomes the front-end of frequency modulated continuous-wave(FMCW) devices that can assist with spatial awareness for automateddriver assist systems, or self-driving vehicles. As shown, the LIDARsystem 100 includes optical circuits 101 implemented on a photonicschip. The optical circuits 101 may include a combination of activeoptical components and passive optical components. Active opticalcomponents may generate, amplify, or detect optical signals and thelike. In some examples, the active optical circuit includes opticalbeams at different wavelengths, one or more optical amplifiers, one ormore optical detectors, or the like.

Free space optics 115 may include one or more optical waveguides tocarry optical signals, and route and manipulate optical signals toappropriate input/output ports of the active optical circuit. The freespace optics 115 may also include one or more optical components such astaps, wavelength division multiplexers, splitters/combiners,polarization beam splitters, collimators or the like. In someembodiments, as discussed further below, the free space optics 115 mayinclude components to transform the polarization state and directreceived polarized light to optical detectors using a PBS.

An optical scanner 102 includes one or more scanning mirrors that arerotatable along respective orthogonal axes to steer optical signals toscan an environment according to a scanning pattern. For instance, thescanning mirrors may be rotatable by one or more galvanometers. Theoptical scanner 102 also collects light incident upon any objects in theenvironment into a return optical beam that is returned to the passiveoptical circuit component of the optical circuits 101. For example, thereturn optical beam may be directed to an optical detector by apolarization beam splitter. In addition to the mirrors andgalvanometers, the optical scanning system may include components suchas a quarter-wave plate, lens, anti-reflective coated window or thelike.

To control and support the optical circuits 101 and optical scanner 102,the LIDAR system 100 includes a LIDAR control systems 110. The LIDARcontrol systems 110 may include a processing device for the LIDAR system100. In embodiments, the processing device may be one or moregeneral-purpose processing devices such as a microprocessor, centralprocessing unit, or the like. More particularly, the processing devicemay be complex instruction set computing (CISC) microprocessor, reducedinstruction set computer (RISC) microprocessor, very long instructionword (VLIW) microprocessor, or processor implementing other instructionsets, or processors implementing a combination of instruction sets. Theprocessing device may also be one or more special-purpose processingdevices such as an application specific integrated circuit (ASIC), afield programmable gate array (FPGA), a digital signal processor (DSP),network processor, or the like.

In some embodiments, the LIDAR control systems 110 may include a signalprocessing unit 112 such as a digital signal processor. The LIDARcontrol systems 110 are configured to output digital control signals tocontrol optical drivers 103. In some embodiments, the digital controlsignals may be converted to analog signals through signal conversionunit 106. For example, the signal conversion unit 106 may include adigital-to-analog converter. The optical drivers 103 may then providedrive signals to active components of optical circuits 101 to driveoptical sources such as lasers and amplifiers. In some embodiments,several optical drivers 103 and signal conversion units 106 may beprovided to drive multiple optical sources.

The LIDAR control systems 110 are also configured to output digitalcontrol signals for the optical scanner 102. A motion control system 105may control the galvanometers of the optical scanner 102 based oncontrol signals received from the LIDAR control systems 110. Forexample, a digital-to-analog converter may convert coordinate routinginformation from the LIDAR control systems 110 to signals interpretableby the galvanometers in the optical scanner 102. In some embodiments, amotion control system 105 may also return information to the LIDARcontrol systems 110 about the position or operation of components of theoptical scanner 102. For example, an analog-to-digital converter may inturn convert information about the galvanometers' position to a signalinterpretable by the LIDAR control systems 110.

The LIDAR control systems 110 are further configured to analyze incomingdigital signals. In this regard, the LIDAR system 100 includes opticalreceivers 104 to measure one or more beams received by optical circuits101. For example, a reference beam receiver may measure the amplitude ofa reference beam from the active optical circuit, and ananalog-to-digital converter converts signals from the reference receiverto signals interpretable by the LIDAR control systems 110. Targetreceivers measure the optical signal that carries information about therange and velocity of a target in the form of a beat frequency,modulated optical signal. The reflected beam may be mixed with a secondsignal from a local oscillator. The optical receivers 104 may include ahigh-speed analog-to-digital converter to convert signals from thetarget receiver to signals interpretable by the LIDAR control systems110.

In some applications, the LIDAR system 100 may additionally include oneor more imaging devices 108 configured to capture images of theenvironment, a global positioning system 109 configured to provide ageographic location of the system, or other sensor inputs. The LIDARsystem 100 may also include an image processing system 114. The imageprocessing system 114 can be configured to receive the images andgeographic location, and send the images and location or informationrelated thereto to the LIDAR control systems 110 or other systemsconnected to the LIDAR system 100.

In operation according to some examples, the LIDAR system 100 isconfigured to use nondegenerate optical sources to simultaneouslymeasure range and velocity across two dimensions. This capability allowsfor real-time, long range measurements of range, velocity, azimuth, andelevation of the surrounding environment. In some exampleimplementations, the system points multiple modulated optical beams tothe same target.

In some examples, the scanning process begins with the optical drivers103 and LIDAR control systems 110. The LIDAR control systems 110instruct the optical drivers 103 to independently modulate one or moreoptical beams, and these modulated signals propagate through the passiveoptical circuit to the collimator. The collimator directs the light atthe optical scanning system that scans the environment over apreprogrammed pattern defined by the motion control subsystem. Theoptical circuits may also include a polarization wave plate to transformthe polarization of the light as it leaves the optical circuits 101. Inembodiments, the polarization wave plate may be a quarter-wave plate ora half-wave plate. A portion of the polarized light may also bereflected back to the optical circuits 101. For example lensing orcollimating systems may have natural reflective properties or areflective coating to reflect a portion of the light back to the opticalcircuits 101.

Optical signals reflected back from the environment pass through theoptical circuits 101 to the receivers. Because the polarization of thelight has been transformed, it may be reflected by a polarization beamsplitter along with the portion of polarized light that was reflectedback to the optical circuits 101. Accordingly, rather than returning tothe same fiber or waveguide as an optical source, the reflected light isreflected to separate optical receivers. These signals interfere withone another and generate a combined signal. Each beam signal thatreturns from the target produces a time-shifted waveform. The temporalphase difference between the two waveforms generates a beat frequencymeasured on the optical receivers (photodetectors). The combined signalcan then be reflected to the optical receivers 104. Configuration ofoptical circuits 101 for polarizing and directing beams to the opticalreceivers 104 are described further below.

The analog signals from the optical receivers 104 are converted todigital signals using ADCs. The digital signals are then sent to theLIDAR control systems 110. A signal processing unit 112 may then receivethe digital signals and interpret them. In some embodiments, the signalprocessing unit 112 also receives position data from the motion controlsystem 105 and galvanometer (not shown) as well as image data from theimage processing system 114. The signal processing unit 112 can thengenerate a 3D point cloud with information about range and velocity ofpoints in the environment as the optical scanner 102 scans additionalpoints. The signal processing unit 112 can also overlay a 3D point clouddata with the image data to determine velocity and distance of objectsin the surrounding area. The system also processes the satellite-basednavigation location data to provide a precise global location.

FIG. 2 illustrates aspects of the optical circuits 200 of a scanningsystem. For example, the optical circuits 200 of FIG. 2 may be part ofoptical circuits 101 of the LIDAR system 100 as shown with respect toFIG. 1 , according to some example implementations. As shown, an opticalsource 202 is configured to provide an optical beam, such as a laserbeam, to passive optical components of a LIDAR system. For example, theoptical source 202 may be a laser source. The optical beam may passthrough the polarization beam splitter (PBS) 214 as non-polarized light.After passing the PBS 214, the optical beam may enter the free spaceoptics 212. In embodiments, the free space optics 212 may include lens210 and a polarization wave plate 208. Lens 210 may be used tofocus/collimate the light. The polarization of the optical beam can betransformed by use of a polarization wave plate 208. The polarization ofthe optical beam will then be transformed to a circular polarization. Inembodiments, the polarization wave plate 208 may reflect a portion ofthe polarized light back toward optical source 202. In some embodiments,a separate mirror, micro-lens array, filter, or reflective coating onthe lens 210 or polarization wave plate 208 may be used. The reflectedportion of the light becomes a local oscillator for interference withreturned light from a target.

Although the present illustration shows a particular arrangement of thelens 210 and polarization wave plate 208 of the free space optics 212,in other embodiments lens 210, polarization wave plate 208 and any othercomponents of the free space optics 212 may be arranged in variousconfigurations. For example, the free space optics 212 may be configuredsuch that the optical beam passes through the polarization wave plate208 to the lens 210. In some embodiments, the lens 210 may also reflecta portion of the polarized light back toward the optical source 202. Insome embodiments, the polarization wave plate 208 may be positionedprior to the PBS 214. The tilt angle of the PBS 214 may then be adjustedsuch that a portion of the optical beam is redirected towards the WGPD204 as the local oscillator signal.

After passing through the polarization wave plate 208, the optical beamis transmitted to the environment and a portion of the pulse may reflectback from one or more objects. For example, the light may be transmittedto the environment by one or more fast scanning mirrors as discussedwith respect to FIG. 1 in a raster pattern. A portion of the reflectedlight may be returned in the direction of the optical source 202 as atarget signal. The free space optics 212 may be configured to combinethe received target signal with the local oscillator signal to produce acombined signal that is spatially aligned and co-propagating. Becausethe combined signal that includes the target signal and the localoscillator signal is polarized, when the combined signal is returned tothe polarized beam splitter 206, rather than passing back to the opticalsource 202, the combined signal is reflected to a waveguidephotodetector 204. The local oscillator signal and the signal from thetarget have interfered to generate the combined signal. Accordingly,there is no need to have the two signals interfere with one another. Thecombined signal can then be used to interpret distance, velocity, orother factors about the environment at the target point.

FIG. 3 illustrates aspects of a LIDAR system 300 in accordance withembodiments of the present disclosure. The LIDAR system 300 may includea photonic chip 302 including one or more components of LIDAR system300. In embodiments, the optical circuits 101, as previously describedat FIG. 1 , are implemented on photonic chip 302. Optical circuits 101may generate an optical beam that is passed through a single-modewaveguide 306 configured for a single-mode propagation of the opticalbeam. In some embodiments, the optical beam may be provided from the SMwaveguide 306 to an optional beam expander 308 a that is configured toenlarge the mode area of the optical beam.

Upon exiting the photonic chip 302, the optical beam may pass through apolarization beam splitter (PBS) 214, as previously described. Inembodiments, a lens (not shown) may be positioned between the photonicchip 302 and the PBS 214 to collimate the optical beam prior to enteringthe PBS 214. The LIDAR system 300 may further include free space optics212 to transform the polarization of the light, reflect a portion of thelight as a local oscillator signal 314, combine the target signal andlocal oscillator signal, etc., as previously described at FIG. 2 .

Upon transforming the polarization of the light, a portion of theoptical beam may be transmitted towards a target 316 via a scanner 312(e.g., optical scanner 102 of FIG. 1 ). When the optical beam hits thetarget 316, a portion of the beam is returned back to the LIDAR system300 as a target signal 318. The target signal 318 is received by thefree space optics 212 where target signal 318 is combined with the localoscillator signal 314, as previously described. The combined signal isthen received by PBS 214 and redirected towards a multi-mode (MM)waveguide 326. In embodiments, the combined signal may be redirectedfrom the PBS 214 to the MM waveguide 326 via one or more fold mirrors316. In some embodiments, prior to entering the MM waveguide 326, thecombined signal may be passed through an optional beam expander 308 b toenlarge the mode area of the combined signal. The combined signal maythen pass through the MM waveguide 326 where the combined signal isreceived by a waveguide photodetector (WGPD) 320. The combined signalcan then be used to interpret distance, velocity, or other factors aboutthe environment at the target point.

FIG. 4 illustrates aspects of a LIDAR system 400 having multiple opticalsources in accordance with embodiments of the present disclosure. TheLIDAR system 400 may include a photonic chip 402 including one or morecomponents of LIDAR system 400. In embodiments, the photonic chip 402may include optical circuits 101 as previously described at FIG. 1 .Optical circuits 101 may include multiple optical sources that generateoptical beams. In embodiments, a first optical beam may have a firstwavelength (e.g., λ1) and a second optical beam may have a secondwavelength (e.g., λ2). Upon generating the optical beams, the opticalbeams may be multiplexed together by multiplexer (MUX) 404 into a singleoutput beam. The multiplexed optical beam may be passed through asingle-mode waveguide 406 configured for a single-mode propagation ofthe optical beam. In some embodiments, the optical beam may be providedto a beam expander 408 a that is configured to enlarge the mode area ofthe optical beam.

Upon exiting the photonic chip 402, the optical beams may pass through apolarization beam splitter (PBS) 414, as previously described. Inembodiments, a lens (not shown) may be positioned between the photonicchip 402 and the PBS 414 to collimate the optical beams prior toentering the PBS 414. The LIDAR system 400 may further include freespace optics 412 to transform the polarization of the optical beams,reflect a portion of the light as a local oscillator signal 422, combinethe target signal 418 and local oscillator signal 422, etc., aspreviously described at FIG. 2 .

Upon transforming the polarization of the light, a portion of theoptical beam may be transmitted towards a target 416 via a scanner 424(e.g., optical scanner 102 of FIG. 1 ). When the optical beam hits thetarget 416, a portion of the beam is returned back to the LIDAR system400 as a target signal 418. The target signal 418 is received by thefree space optics 412 where target signal 418 is combined with the localoscillator signal 422. The combined signal is then received by PBS 414and redirected towards a demultiplexer (DEMUX) 430. The DEMUX 430 may beconfigured to receive the optical beam and redirect a first portion ofthe optical beam to a first location and a second portion of the opticalbeam to a second location based on wavelength. For example, DEMUX 430may redirect a first portion of the optical beam having wavelength λ1 toa first location and a second portion of the optical beam havingwavelength λ2 to a second location. Further details regarding DEMUX 430are described below at FIGS. 5A and 5B.

A first combined signal that includes a first LO signal and a firsttarget signal having wavelength λ1 may be redirected by DEMUX 430towards a first MM waveguide 426 a. In some embodiments, prior toentering the first MM waveguide 426 a, the first combined signal may bepassed through a beam expander 408 b configured to enlarge the mode areaof the first combined signal. The first combined signal may then passthrough the first MM waveguide 426 a where the combined signal isreceived by a first waveguide photodetector (WGPD) 420 a.

A second combined signal that includes a second LO signal and a secondtarget signal having wavelength λ2 may be redirected by DEMUX 430towards a second MM waveguide 426 b. In some embodiments, prior toentering the second MM waveguide 426 a, the second combined signal maybe passed through a beam expander 408 c configured to enlarge the modearea of the second combined signal. The second combined signal may thenpass through the second MM waveguide 426 b where the combined signal isreceived by a second waveguide photodetector (WGPD) 420 b.

FIG. 5A is an illustration of an example demultiplexer 500 in accordancewith embodiments of the present disclosure. In embodiments,demultiplexer (DEMUX) 500 may correspond to DEMUX 430 of FIG. 4 . DEMUX500 may include a dichroic mirror 502 and a fold mirror 504.

The dichroic mirror 502 may be configured to reflect/redirect opticalbeams of a particular wavelength, while allowing optical beams of adifferent wavelength to pass through the dichroic mirror 502. Forexample, dichroic mirror 502 may be configured to redirect optical beamshaving a wavelength of λ1, while allowing optical beams having awavelength of λ2 to pass through the dichroic mirror 502. Referring toFIG. 5A, dichroic mirror 502 is configured to redirect optical beamshaving a wavelength of λ1 while allowing optical beams having awavelength of λ2 to pass through the dichroic mirror 502. Accordingly, afirst portion of the combined signal including the first LO signal andthe first target signal having a wavelength of λ1 is redirected into MMwaveguide 426 a, as previously described at FIG. 4 .

A second portion of the combined signal including the second LO signaland second target signal having a wavelength of λ2 passes through thedichroic mirror 502, where the second portion of the combined signal isredirected by fold mirror 504 into MM waveguide 426 b, as previouslydescribed at FIG. 4 .

FIG. 5B is an illustration of an example demultiplexer 550 in accordancewith some embodiments of the present disclosure. In embodiments,demultiplexer (DEMUX) 550 may correspond to DEMUX 430 of FIG. 4 . DEMUX550 may include a dispersive element 552 and fold mirrors 554 and 556.

The dispersive element 552 may be formed of one or more materials thatare configured to disperse portions of the optical beam at differentangles based on the wavelength of the optical beam. For example,dispersive element 552 may be configured to direct a first portion of anoptical beam having a wavelength of λ1 at a first angle and a secondportion of the optical beam having a wavelength of λ2 at a second angle.Referring to FIG. 5B, the dispersive element 552 is configured to directa first portion of the optical beam having a wavelength of λ1 at a firstangle towards fold mirror 554 and direct a second portion of the opticalbeam having a wavelength of λ2 at a second angle towards fold mirror556. Fold mirror 554 may be configured to redirect the first portion ofthe optical beam into a first MM waveguide 426 a and fold mirror 556 maybe configured to redirect the second portion of the optical beam into asecond MM waveguide 426 b.

Accordingly, a first portion of the combined signal including the firstLO signal and the first target signal having a wavelength of λ1 isredirected by dispersive element 552 to fold mirror 554 and into MMwaveguide 426 a. A second portion of the combined signal including thesecond LO signal and second target signal having a wavelength of λ2 isredirected by dispersive element 552 to fold mirror 556 and into MMwaveguide 426 b, as previously described at FIG. 4 .

FIG. 6 illustrates aspects of the optical circuits 600 of a scanningsystem. For example, the optical circuits 600 of FIG. 6 may be part ofoptical circuits 101 of the LIDAR system 100 as shown with respect toFIG. 1 , according to some example implementations. As shown, opticalcircuits 600 include multiple optical sources 602 a, 602 b and multipleWGPDs 604 a, 604 b. The multiple optical sources 602 a, 602 b and WGPDs604 a, 604 b may provide multiple data points during a single timeinterval. Accordingly, fewer rotations of the fast scanning mirrors mayprovide additional data. The remaining portions of the optical circuits600 may be the same or similar to those described above with referenceto FIG. 2 . For example, the optical circuits 600 may include free spaceoptics 412 having a polarization wave plate 608 to transform thepolarization of light, lens 610 to collimate the light, and the like. Insome embodiments, the alignment of the PBS 414 may be set such that theoptical sources 602 a, 602 b and WGPDs 604 a, 604 b are aligned whenreturned light is reflected. In some embodiments, there may be multipleoptical sources 602 a, 602 b, multiple WGPDs 604 a, 604 b, and alsomultiple PBSs 414. The signals received at each of the WGPDs 604 a, 604b may be analyzed separately to generate distance or velocity data at apoint. In some embodiments, the optical sources 602 a, 602 b may provideoptical beams at different wavelengths.

FIG. 7A illustrates aspects of a LIDAR system 700 having multipleoptical sources generating multiple optical beams in accordance withembodiments of the present disclosure. The components of LIDAR system700 may be similar to the components of LIDAR system 400. However,rather than having two optical sources that each generate one opticalbeam, the two optical sources each generate two optical beams. Forexample, a first optical source may generate a first optical beam havinga first wavelength and a second optical beam having a second wavelength.Similarly, a second optical source may generate a third optical beamhaving a third wavelength and a fourth optical beam having a fourthwavelength.

The optical beams generated by the first optical source may bemultiplexed together by MUX 404 a and the optical beams generated by thesecond optical source may be multiplexed together by MUX 404 b. Themultiplexed beam from the first optical source (e.g., Source 1) and themultiplexed beam from the second optical source (e.g., Source 2) arethen directed towards target 416, as previously described. A targetsignal 418 is received and combined with LO signal 422. The combinedsignal is redirected towards DEMUX 430 by PBS 414. The DEMUX 430 isconfigured to separate the combined signal based on wavelength andoptical source. For example, the DEMUX 430 may separate a first portionof the combined signal that corresponds to a first wavelength (e.g., λ1)from the first optical source (e.g., Source 1) and direct the firstportion of the combined signal towards MM waveguide 426 a. DEMUX 430 mayseparate a second portion of the combined signal that corresponds to thefirst wavelength from the second optical source and direct the secondportion of the combined signal towards MM waveguide 426 b. DEMUX 430 mayfurther separate a third portion of the combined signal that correspondsto a second wavelength (e.g., λ2) from the first optical source anddirect the third portion of the combined signal towards MM waveguide 426c. DEMUX 430 may also separate a fourth portion of the combined signalthat correspond to the second wavelength from the second optical sourceand direct the fourth portion of the combined signal towards MMwaveguide 426 d.

Although described as having two optical sources generating multipleoptical beams, embodiments of the disclosure may utilize any number oflaser sources generating multiple optical beams of differingwavelengths. For example, aspects of the disclosure may be utilized by aLIDAR system having a single optical source that generates two or moreoptical beams of differing wavelengths.

FIG. 7B illustrates aspects of a LIDAR system 750 having multipleoptical sources generating multiple optical beams in accordance withother embodiments of the present disclosure. The components of LIDARsystem 750 may be similar to the components of LIDAR system 700.However, rather than having one set of passive optical circuits utilizedby the multiple optical sources, in FIG. 7B each of the optical sourcesmay have a corresponding set of passive optical circuits. For example, afirst optical source may have a corresponding PBS 414 a, free spaceoptics 412 a and scanner 424 a and a second optical source may have acorresponding PBS 414 b, free space optics 412 b and scanned 424 b.Similarly, each of the optical sources may have a corresponding DEMUX(e.g., DEMUX 430 a and 430 b) that separates a received combined signalbased on wavelength, as previously described at FIGS. 4-5 . For clarity,optional beam expanders are not shown prior to MM waveguides 426 a-d.However, in some embodiments, beam expanders may be positioned onphotonic chip 402 prior to MM waveguides 426 a-d.

Although described as having two optical sources generating multipleoptical beams, embodiments of the disclosure may utilize any number oflaser sources generating multiple optical beams of differingwavelengths. For example, aspects of the disclosure may be utilized by aLIDAR system having a single optical source that generates two or moreoptical beams of differing wavelengths.

FIG. 8 depicts a flow diagram of a method 800 for combining a targetsignal and a local oscillator signal into a multi-mode waveguide inaccordance with implementations of the present disclosure. Inembodiments, various portions of method 800 may be performed by LIDARsystems 100, 300 and/or 400 of FIGS. 1, 3 and 4 respectively.

With reference to FIG. 8 , method 800 illustrates example functions usedby various embodiments. Although specific function blocks (“blocks”) aredisclosed in method 800, such blocks are examples. That is, embodimentsare well suited to performing various other blocks or variations of theblocks recited in method 800. It is appreciated that the blocks inmethod 800 may be performed in an order different than presented, andthat not all of the blocks in method 800 may be performed.

At block 802, an optical source of the LIDAR system generates an opticalbeam towards a target. In embodiments, multiple optical sources maygenerate multiple optical beams. In some embodiments, the multipleoptical beams may have different wavelengths. The generated optical beammay pass through a PBS and free space optics towards a target, aspreviously described. In embodiments, the polarization of the one ormore optical beams may be transformed by a polarization wave plate.

At block 804, the LIDAR system receives a target signal associated witha reflection of the optical beam by the target. The LIDAR system mayalso receive a local oscillator signal associated with a reflection ofthe optical beam by free space optics, as previously described at FIGS.2 and 6 . In embodiments, multiple target signals and multiple localoscillator signals generated by multiple optical sources may be receivedby the LIDAR system.

At block 806, the LIDAR system combines the target signal and the localoscillator signal into a multi-mode (MM) waveguide. In embodiments, freespace optics of the LIDAR system may be configured to combine the targetsignal and the local oscillator signal such that the signals arespatially aligned and co-propogating. The combined signal may beredirected towards the MM waveguide by a PBS and/or one or more foldmirrors. In embodiments, the combined signal may then be provided to awaveguide photodetector via the MM waveguide. In some embodiments, thewaveguide photodetector and the MM waveguide may reside on the samephotonic chip.

In embodiments utilizing multiple optical sources to generate multipleoptical beams, a DEMUX may be utilized to direct a first combined signalhaving a first wavelength into a first MM waveguide and a secondcombined signal having a second wavelength into a second MM waveguide.In some embodiments, the DEMUX may include a dichroic mirror and a foldmirror, as previously described at FIG. 5A. In an embodiment, the DEMUXmay include a dispersive element and one or more fold mirrors, aspreviously described at FIG. 6B.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some embodiments of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present disclosure. Thus, the specific details set forth are merelyexemplary. Particular embodiments may vary from these exemplary detailsand still be contemplated to be within the scope of the presentdisclosure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiments included inat least one embodiment. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.In addition, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or.”

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittent oralternating manner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Moreover, use of the term “an embodiment” or “one embodiment” or“an implementation” or “one implementation” throughout is not intendedto mean the same embodiment or implementation unless described as such.Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. asused herein are meant as labels to distinguish among different elementsand may not necessarily have an ordinal meaning according to theirnumerical designation.

What is claimed is:
 1. A light detection and ranging (LIDAR) apparatus,comprising: an optical source configured to emit an optical beam; apolarization wave plate configured to transform a polarization state ofthe optical beam; a lens configured to collimate the optical beam; freespace optics configured to: receive a first portion of the optical beamas a target signal; receive a second portion of the optical beam fromthe polarization wave plate as a local oscillator signal, wherein thepolarization wave plate comprises a reflector or a coating to return thesecond portion of the optical beam; and combine the target signal andthe local oscillator signal; a set of multi-mode (MM) waveguides; and ademultiplexer comprising a dispersive element, the demultiplexerconfigured to: disperse, via the dispersive element, each respectivewavelength of the combined signal at a corresponding angle; and reflecteach respective wavelength of the combined signal to a corresponding MMwaveguide of the set of MM waveguides.
 2. The LIDAR apparatus of claim1, further comprising: a polarization beam splitter configured to pass afirst polarization state of light through the beam splitter in a firstdirection and reflect a second polarization state of light in a seconddirection different than the first direction.
 3. The LIDAR apparatus ofclaim 2, further comprising: second lensing optics configured tocollimate the optical beam through the polarization beam splitter. 4.The LIDAR apparatus of claim 1, further comprising: a set of waveguidephotodetectors, each waveguide photodetector of the set of waveguidephotodetectors configured to receive the combined signal from arespective MM waveguide.
 5. The LIDAR apparatus of claim 4, wherein theoptical source, the set of waveguide photodetectors and the set of MMwaveguides are positioned on a photonic chip.
 6. The LIDAR apparatus ofclaim 1, wherein the polarization wave plate comprises a quarter-waveplate or a half-wave plate.
 7. The LIDAR apparatus of claim 1, furthercomprising: a multiplexer to multiplex a set of component optical beamsinto the optical beam, wherein each of the set of component opticalbeams has a respective wavelength that corresponds to a respectivewavelength of the combined signal.
 8. The LIDAR apparatus of claim 7,wherein the demultiplexer further comprises: a set of mirrors, each ofthe set of mirrors positioned at the corresponding angle of a respectivewavelength of the combined signal and configured to reflect therespective wavelength of the combined signal to the corresponding MMwaveguide.
 9. A method comprising: generating, by an optical source of alight detection and ranging (LIDAR) system, an optical beam towards atarget; transforming, by a polarization wave plate, a polarization stateof the optical beam; collimating the optical beam; receiving, by freespace optics of the LIDAR system, a first portion of the optical beam asa target signal; receiving a second portion of the optical beam from thepolarization wave plate as a local oscillator signal; combining thetarget signal and the local oscillator signal into a combined signal;and dispersing, via a dispersive element of a demultiplexer, eachrespective wavelength of the combined signal at a corresponding angle;and reflecting, via the demultiplexer, each respective wavelength of thecombined signal to a corresponding MM waveguide of a set of MMwaveguides.
 10. The method of claim 9, further comprising: receiving, ateach of a set of waveguide photodetectors, a respective wavelength ofthe combined signal from a respective MM waveguide.
 11. The method ofclaim 10, wherein the LIDAR system, the set of MM waveguides and the setof waveguide photodetectors are on a photonic chip.
 12. The method ofclaim 9, further comprising: multiplexing a set of component opticalbeams into the optical beam, wherein each of the set of componentoptical beams has a respective wavelength that corresponds to arespective wavelength of the combined signal.
 13. The method of claim12, wherein reflecting each respective wavelength of the combined signalto a corresponding MM waveguide comprises: reflecting, via each of a setof mirrors of the demultiplexer, a respective wavelength of the combinedsignal to the corresponding MM waveguide.
 14. The method of claim 12,wherein reflecting a respective wavelength of the combined signal to acorresponding MM waveguide comprises: directing a first wavelength ofthe combined signal towards a first mirror; and directing a secondwavelength of the combined signal towards to second mirror.